eldo_user

Speed and Accuracy
One Step Relaxation (OSR) Algorithm
and thus OSR cannot perform equally well. Specifically, gate-drain and gate-source leakage
currents can easily prevent the algorithm from working efficiently. For these reasons OSR is
considered a legacy feature, and there are no active developments, enhancements, nor bug
fixing. To perform large-scale digital (CMOS) simulation with a higher performance than Eldo
Classic, consider the ADiT and Eldo Premier solutions.
When the conditions for its applicability are met, the speed and accuracy of OSR are excellent.
In many cases, it is just as accurate as the Newton method, but much faster, and also the growth
of the CPU time with the circuit size is almost linear, which enables the simulation of larger
circuits. Although its capacity is still less than that of fast-SPICE timing simulators such as
ADiT, it provides an interesting point in the speed/accuracy plane.
OSR is not particularly efficient with tightly coupled analog circuits. Better results will be
obtained using the default Newton-Raphson method.
OSR can be activated explicitly or indirectly. The most straightforward way to activate OSR is
to use .option OSR. If this option is set, Eldo attempts to simulate the whole circuit with OSR.
However, if the circuit contains elements that prevent OSR from being effective, or if its
connectivity is such that OSR will fail, Eldo reverts back to the default Newton algorithm, for
part or all of the circuit and will issue a warning.
Note
Eldo is able to simulate a circuit with certain parts handled by OSR, and other parts handled
by Newton. See “Combined OSR/Newton Simulation” below for more information.
OSR and the Notion of Latency
When OSR is used, Eldo places the nodes for which the surrounding nodes do not show any
voltage variation greater than EPS volts into ‘latency’. Thereafter, Eldo effectively bypasses the
calculation of such nodes. This enables significant CPU time savings, especially for large digital
circuits where, often, only a fraction of the nodes are changing over one time step.
Due to the formulation of OSR, latency exploitation is very natural and effective. Latency is
much more complicated to control in a reliable way in the context of the regular Newton
method.
Even with OSR, latency, and above all ‘wake-up from latency’, is always tricky. When slowly
varying signals are applied to a circuit with high capacitances, Eldo may not detect any voltage
variation within one time step, resulting in nodes being placed in latency. Depending on the
tolerances (EPS) this may cause incorrect simulation results. In case latency is potentially the
source of problems, the .OPTION NOLAT command may be used to suppress the use of
latency. If this option is set, Eldo will not attempt to use latency, and will solve all nodes,
whatever their activity.
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Eldo® User's Manual, 15.3